This invention is concerned with a reagent composition and process for desulfurizing blast furnace hot metal external to the furnace itself which provides efficient removal of sulfur from liquid hot metal together with reduced costs for this operation.
The demand for low sulfur liquid iron to be charged to steel refining furnaces continues to increase as requirements for stronger, tougher steels with good formability, weldability and better surface quality expands worldwide. In general, sulfur is an undesirable impurity in high quality steels except in cases where it is intentionally added to improve machinability.
In the past, a number of reagents have been developed and employed for desulfurizing hot metal external to the blast furnace in both torpedo cars and transfer ladles. These may be classified into the following three general categories: (1) sulfur removal by metals; (2) sulfur removal by compounds; and (3) sulfur removal by slags. All of these classes of materials and the manner in which the operations have been conducted have both technical and economic limitations. As yet there is no preferred method of external desulfurization which is universally accepted or practiced. Desulfurization is a complex process that is affected by many interrelated technical factors.
In order to appreciate better the process of sulfur removal from blast furnace hot metal, it is first necessary to consider the effect of some of the elements normally present in the hot metal such as carbon, silicon, phosphorous, aluminum, etc., on the activity coefficient of sulfur in liquid iron. All of these elements increase the activity coeffiecient of sulfur in iron. Carbon and silicon, both of which are present in significant quantities in hot metal increase the activity coefficient of sulfur appreciably. For normal hot metal containing 4.2% C and 1.5% Si, the activity coefficient is about 5. This means that the sulfur activity in this hot metal is five times greater than that expected from the actual sulfur concentration in the metal. Because of this, sulfur removal is much easier from hot metal than from steel which contains less carbon and silicon. Thus, it is logical to assume that if additional carbon is added to a desulfurizing slag reagent, it should make sulfur removal more effective.
With regard to the first method, desulfurization by metals, the ability of a given metal to remove sulfur can be predicted from the free energy of formation of its sulfide together with a knowledge of its solubility in liquid iron. Sodium, potassium, calcium, strontium and barium all have high negative free energies of formation of their sulfides making them good candidates for desulfurization. However, all of these metals have minimal or no solubility in liquid iron. This leaves only two metals, magnesium and rare earths with the required high-negative free energy of sulfide formation plus the required solubility in iron available for hot metal desulfurization. The rare earth metals desulfurize effectively, but a large quantity is required to remove a given weight of sulfur because of their high atomic weight. This plus their much higher cost rules out their use for iron treatment.
With regard to the second method, desulfurization with compounds such as lime, soda ash, calcium carbide, calcium cyanamide, etc. depends upon finding a compound with a free energy of formation of its sulfide which is more negative than the free energy of formation of the compound itself. In the case of burnt lime, CaO, for example, as a desulfurizing agent the free energy of formation of CaO is more negative than the free energy of formation of CaS, and desulfurization cannot proceed unless the oxygen released in the reaction is continuously removed by reaction with carbon, silicon, aluminum or other deoxidizing elements.
With all of the above in mind, optimum desulfurization can be expected to occur only when the thermochemical factors mentioned above are properly combined with the physical conditions employed for desulfurization. For best results the following conditions must be met:
1. The reagent must be thoroughly mixed with the liquid iron from which sulfur is to be removed. The best way of achieving this is by powder injection and/or vigorous stirring.
2. The oxygen potential of the slag/metal system must be low (low FeO and MnO).
3. The reaction products produced must be removed from the system. This requirement emphasizes the necessity of using adequate amounts of fluid basic slags which have good sulfide holding capacity to sequester the sulfide reaction products.
4. There must be effective contact of the slag/metal interface to minimize the time required to remove the sulfide reaction products.
5. Since oxygen forms more stable compounds with the metals, compounds and slag forming materials than sulfur, every attempt must be made to exclude oxygen from the system to avoid reversion of the sulfur from the slag to the metal. This includes oxygen resulting from erosion of the ladle lining as well as oxygen from the reaction, the atmosphere and any oxides present.
It can be seen from these considerations that the development of an efficient reagent for desulfurizing hot metal involves much more than the obvious simple mixing of several materials already known to be able to desulfurize. The reagent composition of this invention combines the desulfurizing action of materials in a manner such that the final result provides a unique material and method of application capable of performing in a manner not achieved by simply adding the individual effects of the materials involved.